1. Field of the Invention
The invention relates to a method of encoding a stream of data bits of a binary source signal into a stream of data bits of a binary channel signal in a transmission system, in particular a system for recording and reproducing a record signal on a record carrier, the bit stream of the source signal being divided into a contiguous sequence of a limited number of permissible source words of variable length, which permissible source words are converted into corresponding channel words, each data bit of a source word resulting in two channel bits of the channel word.
2. Description of the Prior Art
When developing encoding techniques for converting a digital information signal into a binary record signal which is suitable for a specific transmission medium, for example a magnetic record carrier or an optical disc, several factors play a part. Some of these factors are:
1. The bandwidth required for the selected channel code should be compatible with the available band width of the transmission medium. This imposes an upper limit on this required bandwidth, which means that consecutive transitions between the two logic values of the binary channel signal must have at least a specific minimum spacing from each other.
2. Generally a self-clocking channel code is required, i.e. the clock signal required for decoding this channel code can be obtained from the bit stream itself, so that no separate clock synchronization words within this bit stream are needed. This limits the maximum spacing between two consecutive transitions within the channel signal.
3. The decoding time window, which is the magnitude of the time interval within which the presence or absence of a signal transition in the channel code should be detected. The requirements imposed on the transmission medium, for example the record carrier, and the associated equipment become more stringent as this time window becomes smaller.
4. Error propagation, which means that number of erroneously decoded data bits as a result of a single erroneous channel bit. The need to use error correction systems, for example the addition of parity bits, decreases as this error propagation, becomes smaller.
5. Finally, the complexity of the encoding and decoding electronics, which is obviously of great importance in the case of consumer products.
In one of the possible encoding techniques mentioned in the opening paragraph the bit stream of the source signal is divided into a contiguous sequence of a limited number of permissible source words of variable length and these source words are converted into corresponding channel words. This encoding technique is described in for IBM Journal of Research and Development, July 1970, pages 376-383. This article describes a number of codes within the scope of this encoding technique, showing that by means of this encoding technqiue conditions 1 and 2 can be satisfied and that condition 5 can be also met to an acceptable degree. However, this article also shows that this encoding technique may present error-propagation problems. Therefore, when this encoding technique is used generally additional steps are taken to reduce this error propagation, as described in U.S. Pat. No. 3,689,899. However, these steps substantially increase the equipment complexity.